Why was my pan-fried medium-rare ribeye steak moist but chewy?

I cooked my first steak the other day by pan-frying it for about 3 mins each side on med-high, and let it rest for a few minutes afterwards. It was probably about an inch thick. I also lightly salted the pan before frying with rock salt. Anyway, it turned out mostly medium rare with some slightly rare parts mixed in, and it was pretty moist. As I mentioned in the title it was a ribeye steak which didn't appear to have much marbling from my limited experience. Is the possible lack of marbling the reason why it was chewy or did I do something wrong when cooking it? I didnt touch it while it was cooking except to flip it.

Buy Choice Beef (or better) ~~~ Learn how to select the best steak out of the 20 in the meat case ~~~ Buy a thermometer and cook to your desired degree of doneness by whatever method you choose. ~~~ To answer your question....I question that a 1" thick rib-eye cook on medium heat for 3 minutes per side was "medium rare" ....Really no way of knowing...that's what a thermometer is for.

Definitley the quality of the cut. If it was a rib eye without much marbling, it was proably select. Where did you buy it? How much per pound? Also, different cuts of rib eye can affect quality, as can aging. Moral of the story is that cooking method is only part of the deal.

Find yourself a good local buthcer and ask for their advice. Ask for choice cuts, look for a thick flap on the edge (I learned this from another hound on here, great tip), and ask them if the steak was aged. I pay around $21 a pound for a quality, aged rib eye.

$21 a pound, huh? I'll have to remember to do that the next time I win the lottery. All kidding aside, is it possible to get a nicely marbled steak with grass-fed and -finished beef? Or will everything be chewy?

It's totally possible. You just have to find the right rancher/butcher. I can count on one hand the times the number of that combo I've come across, but heck, that just inspires loyalty. Learn how to track down your local small ranchers and talk to them. Some care, some don't. I can get a FAB grassfed flatiron steak big enough for 4 (reasonable portions; 2 if you're being greedy, like me) at my local farmers market for $10. Sometimes you will find that to your taste, you want that grain finishing on a grassfed cow. That's still MUCH healthier meat than the grocery store, and you can make sure that cow never saw a feedlot.

Veter is totally right. Track down your local butchers and ranchers. I live in the SF area and we have Marin Sun Farms, Prather Ranch and a lot of Niman products here. Also, at our local farmer's markets I find smaller ranchers selling too. Ask around for butchers who do their own dry aging too. There is a big difference between grass-fed, grass-fed & corn finished, and corn fed. The $21 a pound is what I expect from a specialty butcher for a local, dry aged, rib-eye, but you can find a quality cut in the $12-$18 a pound range too. It just depends on what's available in your area.

Almost certainly the grade of beef. Marbling seems to add not just succulence but tenderness (to some degree). The Fairway by me always has some prime grade beef on sale in the $8-$10 a pound range (for ribeye, strip, porterhouse, etc..), so I wait for those.

But even if you shop for Choice grade, you can do well. There is a huge variation in Choice beef, from "almost should have been graded select" to "wow, almost could have been graded prime." Look for marbling - pick through the bin!

I also note that Cooks Illustrated found that cooking a large semi-tough roast low and slow, then finishing with a pan sear, resulted in improved tenderness. So if you have a cheap grade of beef, that might be the way to go. I prefer a char on the outside, so I always try for better grade, high heat.